This invention relates generally to stabilizing the frequency of one laser oscillator with respect to another laser oscillator and, more particularly, to the use of a polarized feedback signal to stabilize a slave laser oscillator with respect to a master laser oscillator.
The application of diode-laser-pumped monolithic Nd:YAG lasers to the injection seeding of laboratory-scale Nd:YAG oscillators is rapidly becoming the preferred method for establishing single-mode operation of these laser systems. The resultant elimination of mode-beating-induced temporal modulation would significantly enhance the precision of almost every optical measurement made with Nd:YAG lasers. The teachings of this invention should find general application to other lasers, as well. In spectroscopic applications, such as coherent Raman and sum- or difference-frequency mixing, the narrow bandwidth available form a single-mode oscillator also becomes attractive. In these latter cases, stable frequency operation may be as important as single-mode operation.
The prior art shows several different approaches to this problem. For example, U.S. Pat. No. 4,455,657 of Robert Byer observed that an injection-seeded high-gain laser had an output frequency determined by a selected cavity mode of the high-gain laser resonator, not the injected frequency of the master oscillator. This knowledge was adapted for long-term frequency stabilization by Y. Park et al., IEEE Journal of Quantum Electronics, Vol. QE-20, No. 2, February 1984, wherein the output frequency of both master and slave oscillators was locked to one side of the transmittance curve of a reference interferometer, the frequency of each oscillator being adjusted by adjusting the cavity length.
Another system is shown by R. Teets, IEEE Journal of Quantum Electronics, Vol. QE-20, No. 4, April 1984, wherein a small detuning between master and slave oscillators is maintained by an optical signal from the master oscillator that increased in magnitude when the master oscillator was resonant with the slave oscillator.
A third system is shown by L. Rahn, Applied Optics, Vol. 24, No. 7, April 1985, wherein the timing of an optical signal from a slave oscillator is used to control the cavity length to maintain single-mode operation.
A problem faced by Park is that his system required a third temperature-stabilized cavity in the feedback loop. A problem faced by the other systems is that the feedback signal does not contain information necessary to indicate the sign of the required correction. As a result, a drift of the control signal from a maximum does not tell the control circuitry which direction the correction should be applied to return to the desired condition. Teets overcame this problem by detuning his device. Rahn overcame the problem by dithering his master oscillator signal at the expense of absolute frequency stability.